When oxygen is taken into the bloodstream, the oxygen dissolves into the blood itself. The only thing is that the blood has hemoglobin and hemoglobin molecules are the ones that oxygen molecules attach to to be carried off and transported to the different parts of the body. The hemoglobin molecules contain 4 subunits which are two alpha subunits and two beta subunits. Each subunit contain iron that an oxygen molecule attaches to which makes up to 4 oxygen molecules being carried by a hemoglobin molecule. This is also the reason why oxygenated blood are brighter in color than deoxygenated blood appear to be in darker red.